Loss of hearing has a serious impact on quality of life. Human ingenuity has been directed to addressing the problem of hearing loss, primarily by way of amplification devices. The confluence of modern micro-engineering and medical advances led to the development of aural implants that could directly convert sound waves to electrical impulses that could compensate for hearing loss.
On 1 Aug. 1978 Melbourne resident Rod Saunders became the first person in the world to receive a multi-channel cochlear implant. The cochlear implant is an electrical device that directly converts sound to electrical signals that directly stimulate the auditory system through an array of electrodes wound through the cochlea of a patient. Cochlear implants typically employ 24 electrodes although modern devices have been developed with up to 128 electrodes.
For people already functional in spoken language who lose their hearing, cochlear implants can be a great help in restoring functional comprehension of speech, especially if they have only lost their hearing for a short time. Cochlear implants (CI) have also been found to be useful in young children who have been born with significant hearing loss. In fact, cochlear implants have been provided to children as young as 4 months old.
Approximately half of the people who have received a cochlear implant are adults. Implantation into post-lingual adults, who developed speech before they lost their hearing, is particularly challenging as the electrodes on the implant must be aligned with the frequency specific areas of the patient's cochlea. In particular, it is important that the cochlear implant is inserted deeply enough into the cochlea so as to stimulate the lower frequency portions of the cochlea whilst avoiding damage to the cochlea wall itself. Damage to the cochlea wall may adversely affect residual hearing. Therefore, a perimodiolar insertion of implants is often preferred as they can be inserted more deeply into the scala timpani and closer to the modiolus.
A further problem relates to people with some residual low frequency hearing who are fitted with both a cochlear implant and an acoustic hearing aid, which is often referred to as bimodal amplification. A problem with bimodal amplification is that the sounds travel faster through the cochlear implant as electrical stimuli than the amplified sounds from the acoustic hearing aid. The result is that the sounds are received by the brain at fractionally different times which can lead to difficulties in word recognition. This is especially the case during binaural hearing, where the relative timing between the ears is important for source localisation, and is required for accurate sound perception, e.g., recognising speech in a noisy environment.
In addition, for patients with a partial hearing impairment, it is highly desirable to align the electrodes of the implant with the hair cells (or associated spiral ganglion) that have been damaged or lost. In this way, the electrodes stimulate the appropriate area on the cochlea, thus complementing the patient's remaining hair cells that are still functioning. Therefore, both accurate positioning and mapping of the implant's electrodes can assist effective rehabilitation of hearing loss. Furthermore, many patients who have low frequency residual hearing and are fitted with both a Cochlear implant and an acoustic hearing aid only require a certain range of frequencies to be amplified.
Currently cochlear implants are positioned by pushing the electrodes as far into the cochlea as considered necessary by the physician. In general, there is no attempt to accurately align the electrodes with particular parts of the cochlea. There have been modifications to the conventional linear cochlear electrode arrays to take a curved shape within the ear (see U.S. Pat. No. 5,578,084 assigned to Cochlear Ltd and University of Melbourne), but these variations still do not attempt any form of accurate positioning of the electrode array.
An electric frequency-place function—or better termed electrode-pitch function—is therefore of high importance for the design of the signal processing strategies implemented in cochlear implant speech processor devices. A close match of spectral information in the signal to the electrodes with the corresponding pitch perception of the patient contributes to a better acceptance of the sound of a cochlear implant and enhances the representation of spectral information (U. Baumann, A. Nobbe, The cochlear implant electrode-pitch function, Hearing Research 213 (2006) 34-42).
Currently most cochlear implants have 22 stimulating sites along their length. However, some newer implants host up to 126 stimulating sites, potentially giving greater tonal range and better frequency perception. In each case the remaining two electrodes are used as reference electrodes. Whilst implants with more electrodes could, in principle, deliver an improved perception of speech and other sounds in general, the previously described limitations of methods to position the implant mean that there is currently little perceived benefit to implants with a larger number of electrodes.
A study to relate the pitch of high-rate electrical stimulation delivered to individual cochlear implant electrodes to electrode insertion depth and insertion angle is reported in the Journal of the Association for Research in Otolaryngology (JARO, 2007, Vol 8 pp 234-240) to Dorman et al. This publication studied the results of a patient with a cochlear implant in one ear and some hearing in the other ear. The patient was asked to match tones applied to the implanted ear to tones applied to the non-implanted ear so that the implant could be programmed to match the pitch perception in each ear. This publication discloses that average data may be used to set up signal processors for implant patients however the disadvantage is that it would not be matched to a specific patient. Furthermore, the method attempts to match the pitch perception between the hearing ear and an implanted ear rather than matching a cochlear implant to the cochlear in which it is implanted.
US patent publication 2006/0100672 to Litvak, discloses a method of pitch matching cochlear implants inserted into both ears. However the method does not allow the cochlear implant to be optimised for an individual patient only to optimise the pitch perception of each cochlear implant, in each ear, to be similar.
International patent publication no WO9743871 discloses a method of mapping each electrode of a cochlear implant to a range of frequencies depending on where the cochlear is inserted into the cochlear. However this method is not based on residual hearing of an individual patient, rather it is based on image data obtained from a CT scan.
Therefore, there are three significant limitations that impede clinical use of cochlear implants for post-lingual adults with partial hearing loss:    1. The large inter-subject variation in cochlear physiology and cochlear implant insertion depth leads to uncertainty in the mapping/alignment of electrodes to the appropriate frequency/pitch specific cochlea/spiral ganglion nerve fibres for that individual patient;    2. The limited number of electrodes in most current implants limits the quality of speech and sound perceived by the patient; and    3. The lack of compensation for the timing differences between the arrival of acoustic and electrical stimuli when applied to patients with bimodal amplification.